In recent years, there has been an increasing demand for polarization films partly because of spreading use of liquid crystal display (LCD) devices in various apparatus, such as word processors, personal computers and liquid crystal televisions, and partly because of a trend for greater display areas of LCD devices. Hitherto, polyvinyl-alcohol-type (PVA-type) polarization films have been used in liquid crystal display devices. However, improvements are required to attain higher polarization performance and durability.
PVA-type polarization films are generally categorized into the following types: (1) an iodine-type film in which iodine is adsorbed and oriented on a film material, (2) a dye-type film in which a dichroic dye is adsorbed and oriented on a film material, and (3) a polyene-type film obtained by partly dehydrating a film material.
Among these polarization films, the iodine-type film is typically used because it is superior in its polarization performance as compared with other types of polarization film. Unfortunately, however, the iodine-type polarization film is not resistant to moisture and heat.
In order to overcome this problem, Japanese Kokai No. 56-48601 proposed a method in which a film having iodine adsorbed and oriented thereon is impregnated with cobalt ions. Japanese Kokai No. 62-18030 proposed a method in which a film having iodine adsorbed and oriented thereon is impregnated with nickel ions. These proposals, however, have failed to improve resistance to moisture and heat to an appreciable level, so that the iodine-type film can have only limited use due to inferior humidity and heat resistance despite its superior polarization performance.